At some point, most dialysis patients will stop urinating, but the rate of decline in urination is different for each patient and is dependent upon the type of kidney disease as well as the type of dialysis modality employed. Generally, patients receiving hemodialysis (HD) lose their ability to urinate faster than patients receiving peritoneal dialysis (PD). This decline in urinary output has various implications on patient management and survival.

Patients who continue to urinate while on dialysis (HD or PD) are said to have “residual renal function” (RRF), and this factor is an important consideration in their management. Dietary restrictions on sodium and fluid intake can be liberalized to some degree. Depending on the degree of RRF, less dialysis therapy may be indicated. Nevertheless, close monitoring is needed because kidney function tends to decline over time. Recognition and acceptance of this fact greatly facilitates individualization of dialysis therapy. Studies also suggest dialysis patients, who have RRF, have a greater likelihood of survival than patients who do not. In PD patients, RRF declines steadily by 1.2-2.8 percent per month, irrespective of the underlying kidney disorder that initially led to dialysis2. This survival advantage falls in tandem with declining urinary output.

Similar survival benefits have been shown for HD patients. A Netherlands trial (NECOSAD-2) showed a 56 percent reduction in death in HD patients who had RRF3,4. Similar results were shown in a study conducted at Brown University5. HD patients lose RRF at a faster rate than PD patients; about 5.8-7.0 percent per month, irrespective of the underlying kidney disorder2.

Although PD patients lose RRF at a slower rate than HD patients, the likelihood of survival on either dialysis modality over a five-year period is comparable. Therefore, it would not be advantageous to change from HD to PD despite the differences in RRF.

For those on dialysis, significant RRF permits patients’ a degree of denial about the severity of their kidney disease. As long as a patient has some RRF, it permits the notion that kidney function may recover. As such, a patient may deny that dietary restrictions and dialysis prescriptions are fully applicable to their situation. The consequences of this denial can result potentially in significant morbidity. Conversely, the loss of RRF forces a patient to fully confront the stark reality that he/she is dependent on dialysis and must be fully engaged in appropriate lifestyle and dietary modifications.

Recommendations for stopping the decline in RRF are limited. It is not clear why RRF decreases once patients start dialysis, nor why there is a difference in RRF between HD and PD patients. However, it is safe to assume in most instances there is inexorable progression of the underlying disease process itself. Some general guidelines to slow the decline in RRF include:
1) Vigorous blood pressure control, 2) The use of angiotensin-converting enzyme inhibitors (ramipril, enalapril, lisinopril) and angiotensinreceptor blockers (losartan, valsartan, candesartan), and 3) Avoidance of kidney-toxic agents (non-steroidal anti-inflammatory agents, aminoglycoside antibiotics, iodinated contrast agents used for Cat Scans)2. Because each patient is different, it is important you speak with your physician regarding the applicability of these recommendations to your situation.

More research needs to be done to learn the importance of RRF in patient survival.
Generally, patients who begin dialysis will notice a steady decline in urination (and RRF) at varying rates. Survival is improved if RRF is preserved for patients receiving PD or HD, but RRF alone does not make one form of dialysis better than the other. With additional research, we will know if it is important to direct our therapies toward preserving RRF at all.

References:
1. Bargman JM and Golper TA. The Importance of residual renal function for patients on dialysis. Nephrology Dialysis Transplantation, 2005. Volume 20: pp. 671-673.
2. Jansen MAM, et.al. Predictors of the rate of decline of residual renal function in incident dialysis patients. Kidney International, 2002. Volume 62: pp. 1046-1053.
3. Termorshuizen F, et.al. Relative Contribution of Residual Renal Function and Different Measures of Adequacy to Survival in Hemodialysis Patients: An analysis of the Netherlands Cooperative Study on the Adequacy of Dialysis (NECOSAD)-2. Journal of the American Society of Nephrology, 2004. Volume 15: pp. 1061-1070.
4. Termorshuizen F, et. Al. The relative importance of residual renal function compared with peritoneal clearance for patient survival and quality of life: an analysis of the Netherlands Cooperative Study on the Adequacy of Dialysis (NECOSAD)-2. American Journal of Kidney Diseases, 2003. Volume 41: pp. 1293-1302.
5. Shemin D, et.al. Residual Renal Function and Mortality Risk in Hemodialysis Patients. American Journal of Kidney Diseases, 2001. Volume 38: pp. 85-90.

James Bailey, MD, is a professor of medicine in the division of nephrology at  Emory University and head of the renal fellowship program.

Tejas Desai, MD, is a Nephrology Fellow in the Emory University Division of Nephrology. His academic interests focus on Internetbased medical education for physicians in-training and dialysis patients.

This article originally appeared in the November 2008 issue of aakpRENALIFE.

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